1. Field of Endeavor
The present invention relates to energy storage management systems including advanced battery management systems, systems for managing other energy storage systems, and systems for managing lithium ion and other batteries.
2. State of Technology
Lithium-ion batteries are proven technology, and are leading candidates for terrestrial electric vehicles, back-up power in airplanes and many other uses. Virtually all modern cellular telephones and portable computers use lithium-ion batteries for energy storage. Other important applications of this technology include emerging electric vehicles, such as the Tesla sports car, various autonomous underwater vehicles (AUVs), manned underwater vehicles (UUVs), the Mars Rover, and large laser systems.
This technology has also enjoyed limited but successful use in autonomous underwater vehicles used for oceanographic research. Unfortunately, lithium ion batteries have been plagued by a history of significant safety incidents, with some causing serious human injury and property damage (loss of commercial cargo plane, for example). The lithium-acid battery may prove to be relatively expensive, has safety issues that must be dealt with, but has exceptional performance characteristics, that make it a leading candidate for consideration. Designs would have to emphasize safety, thermal management during charge and discharge, and enhanced battery management systems.
Codes for BMS control algorithms have unique attributes, in that they enable the simultaneous solution of those equations that account for the flow of fluid and heat, chemical reactions, current flow and mechanical stress. However, the equations that describe electrode kinetics and ionic transport have not yet been integrated into the code.
The modern lithium-ion battery has: an anode that consists of a graphite-based active material (Li—C6) with carbon filler and PVDF binder coated onto a copper foil current collector; a cathode that consists of a transition metal oxide or iron phosphate (Li—NiO2, Li—CoO2, Li—MnO2, or Li—FePO4) active material with a PVDF binder coated onto an aluminum foil current collector; a microporous porous polyethylene separator, and an electrolyte consisting of a mixed organic carbonate solvent (EC:DMC:DEC) and LiPF6 salt. The liquid cylindrical or prismatic cells are contained in a hermetically sealed metal can, while polymer-gel cells are contained in a soft aluminum-polyethylene laminate package, with thermally laminated seams. In the case of the polymer-gel cell, the polyethylene separator is usually coated on both sides with porous PVDF layers.
This battery can operate from −40 to +60 degrees Centigrade. The open-circuit voltage is 4.1 V, with operation between 4.0 and 3.0 V (possibly as low as 2.8 V). The specific power, power density, specific energy and energy density are 1100-74 W/kg, 2270-147 W/L, 75-182 Wh/kg, and 139-359 Wh/L, respectively. The cycle life of the best state-of-the-art lithium-ion batteries can be as great as 1500 cycles (to 80% of the original capacity). However, poorly constructed cells can have much shorter lives (300 cycles representing poorer cells). Based upon published data, the cost of energy storage is believed to be approximately $300 per kilowatt-hour (though some quote $1000 per kilowatt-hour).
One key advantage of such flow batteries is the ability to scale the batteries capacity linearly with the size of the reservoirs used for storing the anolyte and catholyte. Other advantages include thermal management, the use of abundant raw materials, efficiency, and the relative ease of construction. The ZnBr battery was patented over 100 years ago, but has never enjoyed widespread commercialization. Technical problems have included the formation of zincdendrites during repeated charging and discharging, which can lead to internal shorts within the cell, and the relatively high solubility of Br in the aqueous electrolyte required by the Zn electrode.
The modern lithium ion battery was developed to overcome safety problems encountered with early rechargeable batteries with metallic lithium anodes. Metallic lithium can react with a wide variety of polymeric materials involved in cell construction, including but not limited to fluorinated polymers such as Teflon. In contrast, the lithium ion battery involves two intercalation electrodes which serve as “nanoscale parking garages” for reduced metallic lithium atoms, thereby avoiding the presence of free metallic lithium in the cell. The use of these two intercalation electrodes, with lithium being shuttled from one parking garage to the other is known as the “rocking chair mechanism” which the intercalation cathode is usually a transition metal oxide or iron phosphate, and the intercalation anode is usually a natural or synthetic graphite. In some cases, lithium alloys such as Li—Sn or Li—Si are used in lieu of graphite.
One problem encountered with advanced battery systems, including lithium ion batteries which rely on the formation of lithium-intercalation compounds at both electrodes, is the plating of dangerous metallic lithium on either the anode active material (graphite) or cathode active material (transition metal oxide or iron phosphate) during repeated cycling. This problem can be exacerbated by attempts to quickly recharge the battery, which will be a temptation in automotive applications. Who wants to wait an hour or two at the filling station to refuel their car? If there is a failure to maintain good contact between adjacent pressure with uniform stack.
The avoidance of lithium plating requires precise understanding of the primary and secondary current distributions inside individual cells, not only at the electrode scale, but also on the length scale of individual micron-sized particles of active material, and on the scale of interatomic spacing in the intercalation compounds that are formed. In addition to modeling the other complexities of the batteries and battery packs (series-parallel strings of individual cells), which include thermal transport and mechanical stresses, it is necessary to make precise predictions of the current and potential distribution between the anodes and cathodes, as well around the individual particles of active material on both electrodes. The physics that must be well understood before predictable and reliable battery packs can be designed include: battery chemistry of nominal charge/discharge; abnormal ageing at a defect (local chemistry, heat, voltage, stress); electric fields and current flow within the cell; heat generation and cell cooling, thermal run-away; convection fluid flow within the electrolyte; external coolant flow; stress and material failure due to volumetric changes during charge/discharge cycle; chemical deflagration of run-away battery; dynamic structural failure of run-away battery cell and battery system; in principle, modern computational modeling could be applied to the design of high performance batteries and battery packs, to help ensure that robust, thermally-stable systems have been built.
Safety is the leading show-stopper for large Li-ion cells, and the battery packs built from those cells. Despite decades of conventional safety testing serious problems remain. Lithium-ion explosions and fires occur frequently in both products and manufacturing plants. In regard to electric vehicle applications, statistics indicates that 1 in every −30,000 electric vehicles with a lithium ion battery pack will burn and/or explode. Given that an electric vehicle with a 100-mile range is capable of releasing a quantity of energy equivalent to −500 sticks of dynamite, this is alarming.
As a lithium ion battery begins to undergo heating, which can be caused by ohmic heating, internal shorting, or the application of heat from outside the cell, a sequence of chemical reactions occur within the Li-ion system, ultimately leading to thermal runaway. During such catastrophic events, numerous chemical reactions begin occurring sequentially. While the inside of the Li-ion cell is oxygen-free, enough oxygen can be liberated from the decomposition of the transition metal oxide active material in the cathode during thermal runaway, regardless of the initiating event, to support limited combustion of the organic carbonate solvents in the electrolyte. Localized internal shorts quickly drive such high performance energy storage systems into thermal runaway, with subsequent propagation to other lithium-ion cells in the battery pack, with further propagation to other packs within the system if they exist.
International Patent Application No. WO 2010/025761 for a system for fire protection provides state of technology information including the following information: “The batteries of the backup power thus contain high amount of energy, and a failing battery cell, e.g. by external or internal short circuit or overload, will quickly become very hot. The heat emitted from the failing cell will heat up an adjacent battery cell, which in turn will heat up the next cell and so on, and this of course constitutes a huge fire hazard. As an example, Li-ion battery cells exceeding a critical temperature may result in opening of the cell, known as venting of the cell, with a release of highly inflammable gases that can easily catch fire. If this happens there is a large risk of the whole battery storage system being destroyed.” The disclosure of International Patent Application No. WO 2010/025761 is incorporated herein in its entirety for all purposes by this reference.